Lithium secondary batteries are attracting attention as high-capacity power sources, and are considered to be promising particularly as power sources for portable devices. A commonly used lithium secondary battery includes an electrode group, a non-aqueous electrolyte and a battery case housing the electrode group and the non-aqueous electrolyte. The electrode group includes a positive electrode, a negative electrode and a porous resin membrane interposed between the electrodes. The porous resin membrane serves to provide electronic insulation between the positive electrode and the negative electrode, and to retain the non-aqueous electrolyte. For example, the positive electrode includes lithium cobaltate as its active material, whereas the negative electrode includes graphite as its active material.
At present, studies aiming at providing even higher capacity for lithium secondary batteries are being carried out. However, the theoretical capacity of lithium cobaltate used as the positive electrode active material is about 150 mAh/g, and that of graphite used as the negative electrode active material is about 370 mAh/g. These theoretical capacities cannot be considered very high. Thus, it is difficult to stably supply lithium secondary batteries including an electrode group with an energy density of not less than 700 Wh/L. Therefore, the use of active materials with high theoretical capacities are being investigated.
For example, there has been proposed a lithium secondary battery including an electrode group in which Si is used as the negative electrode active material (Japanese Laid-Open Patent Publication No. Hei 7-29602). There has also been proposed to set the end-of-charge voltage of a lithium secondary battery to high so as to increase the utilization rate of the positive electrode active material, thereby attaining a high capacity (Japanese Laid-Open Patent Publication No. 2005-85635). In addition, the energy density of a lithium secondary battery can be increased by using a lithium-containing composite oxide containing Ni element as the positive electrode active material.